This invention relates to methods of forming FLASH memory, to methods of forming FLASH memory and SRAM circuitry, and to etching methods.
Memory is but one type of integrated circuitry. Some memory circuitry allows for both on-demand data storage and data retrieval. For example, memories which allow both writing and reading, and whose memory cells can be accessed in a random order independent of physical location, are referred to as random-access memories (RAM). Read-only memories (ROMs) are those in which only the read operation can be performed rapidly. Entering data into a read-only memory is typically referred to as programming, and the operation is considerably slower than the writing operation utilized in random-access memory. With random-access memory, information is typically stored with respect to each memory cell either through charging of a capacitor or the setting of a state of a bi-stable flip-flop circuit. With either, the stored information is destroyed when power is interrupted. Read-only memories are typically non-volatile, with the data being entered during manufacturing or subsequently during programming.
Some read-only memory devices can be erased as well as written to by a programmer. Erasable read-only memory typically depends on the long-term retention of electronic charge as the information storage mechanism. The charge is typically stored on a floating semiconductive gate, such as polysilicon. One type of read-only memory comprises FLASH memory. Such memory can be selectively erased rapidly through the use of an electrical erase signal.
A FLASH memory cell typically comprises a single floating gate transistor. For multiple storage cells, such as used in large semiconductor memories, the storage cells of the memory are arranged in an array consisting of rows and columns. The rows are typically considered as comprising individual conductive gate lines formed as a series of spaced floating gates received along a single conductive line. Source and drain regions of the cells are formed relative to active area of a semiconductor substrate, with the active areas being generally formed in lines running substantially perpendicular to the lines of floating gates. The sources and drains are formed on opposing sides of the lines of floating gates within the active area with respect to each floating gate of the array. Thus, lines (rows) of programmable transistors are formed.
Electrical connections are made with respect to each drain to enable separate accessing of each memory cell. Such interconnections are arranged in lines comprising the columns of the array. The sources in FLASH memory, however, are typically all interconnected and provided at one potential, for example ground, throughout the array. Accordingly, the source regions along a given line of floating gates are typically all provided to interconnect within the substrate in a line running parallel and immediately adjacent the line of floating gates. These regions of continuously running source area are interconnected outside of the array, and strapped to a suitable connection for providing the desired potential relative to all the sources within the array. Accordingly, prior art techniques have been utilized to form a line of continuously running implanted source material within the semiconductor substrate and running parallel with the floating gate word lines.
In a principal technique of achieving the same, the substrate has first been fabricated to form field oxide regions by LOCOS. The fabrication forms alternating strips of active area and LOCOS field oxide running substantially perpendicular to the floating gate word lines which will be subsequently formed. Thus running immediately adjacent and parallel with the respective word lines will be an alternating series of LOCOS isolation regions and active area regions on both the source and drain sides of a respective line of floating gates. After forming the lines of floating gates and to provide a continuous line of essentially interconnected source regions, the substrate is masked to form an exposed area on the source side of the respective lines of floating gates. The LOCOS oxide is then selectively etched relative to the underlying substrate. This leaves a series of spaced trenches along the lines of floating gates the result of removal of oxide from the previously oxidized substrate which formed the LOCOS regions.
Non-recessed LOCOS in fabrication of FLASH memory in this manner is typically very shallow relative to the semiconductor substrate (i.e., less than 1500 Angstroms deep). This leaves a gradual, almost sinusoidal, undulating surface of exposed semiconductor substrate running in lines substantially parallel and immediately adjacent the lines of floating gates on the desired source side. With the gently sloping sidewalls of the trenches or recesses left by the LOCOS oxide removal, one or more source ion implant steps are conducted through the mask openings of the remaining photoresist layer. The result is formation of a continuously and conductively doped source line within the semiconductor substrate immediately adjacent the line of floating gates.
Circuitry fabrication and isolation of adjacent circuitry within a semiconductor substrate can also be achieved with a trench isolation that is different from LOCOS. For example, trenches can initially be etched within a semiconductor substrate and subsequently filled with an insulating material, such as high density plasma deposited oxide. Such trenches can and are sometimes made considerably deeper relative to the outer substrate surface as compared to the oxidation depth of LOCOS. Accordingly, the etching typically produces elongated, deeper and straighter sidewalls than LOCOS.
This invention comprises methods of forming FLASH memory, methods of forming FLASH memory and SRAM circuitry, and etching methods. In one implementation, a method of forming an array of FLASH memory includes forming a plurality of lines of floating gates extending from a memory array area to a peripheral circuitry area over a semiconductor substrate. In a common masking step, discrete openings are formed over a) at least some of the lines of floating gates in the peripheral circuitry area, and b) floating gate source area in multiple lines along at least portions of the lines of floating gates within the memory array area. In one implementation, a line of floating gates is formed over a semiconductor substrate. A conductive line different from the line of floating gates is formed over the semiconductor substrate. In a common masking step, discrete openings are formed to a) at least one of the conductive line and the line of floating gates, and b) floating gate source area of multiple transistors comprising the line of floating gates along at least a portion of the line of floating gates.
In one implementation, a method of forming FLASH memory and SRAM circuitry includes forming a line of floating gates over a semiconductor substrate and an SRAM gate over the semiconductor substrate. In a common masking step, discrete openings are formed over a) the SRAM gate, and b) floating gate source area of multiple transistors comprising the line of floating gates along at least a portion of the line of floating gates. In one implementation, in a common masking step, a local interconnect opening is formed over and extends from the SRAM gate to a source/drain area in an SRAM cell area and an elongated source implant opening is formed over floating gate source area of multiple transistors comprising the line of floating gates along at least a portion of the line of floating gates.
The invention in one implementation comprises, in a common etching step, etching insulative material over an SRAM gate to expose conductive material of the SRAM gate and insulative material over a semiconductor substrate in a line proximate a line of floating gates to expose the semiconductor substrate. The invention in one implementation comprises, in a common etching step, etching insulative material over an SRAM cell source area to expose semiconductive material of the SRAM cell source area and insulative material over a semiconductor substrate in a line proximate a line of floating gates to expose the semiconductor substrate. The invention also comprises, in a common etching step, etching insulative material over a conductive line to expose conductive material of the line and insulative material over a semiconductor substrate in a line proximate a line of floating gates to expose the semiconductor substrate.